England's Root wants to convert more fifties into hundreds

England batsman Joe Root has identified the need to be more assertive to convert his half-centuries to three figure scores, starting with the third test against Sri Lanka at Lord's on Thursday.

The 25-year-old has been dismissed 13 times between the scores of 50 and 98 since 2015, and was most recently caught on 80 against Sri Lanka in the second test, which he described as a "car crash".

Root, who is ranked second in the world test rankings behind Australia's Steven Smith, said he should have doubled the nine test centuries he has amassed in his career.

"Probably double (number of centuries) if you look at the amount of fifties. You are always striving to get better. Over the last couple of months it has been very frustrating," Root told British media.

"I am contributing quite consistently and I feel like I'm playing well, really well.

"But you don't win games with seventies and eighties and starts, so you want to make sure you cash in when you get those opportunities.

"There have been a few decent deliveries but basically and mainly it has been batsman error that needs to be addressed in practice."

England head into the third test against Sri Lanka with an unassailable 2-0 series lead, and Root said he hoped to target a big score.

"This week I'll just have to make sure if the opportunity arises that history doesn't repeat itself," he added.

"It's an annoyance I want to put right and make sure over the rest of the summer if I get in and pass 50 I'm not throwing it away."